The present invention relates to a method of forming a resist pattern in the lithography process performed in manufacturing a semiconductor device or an exposure mask.
To manufacture semiconductor devices of high integration density, fine resist patterns must be formed. To minimize the manufacturing cost of the devices, it is desired that the exposure apparatuses that have been used for the past thirty years should form resist patterns as fine as they can. Further, resist patterns should be formed to satisfy two requirements. First, they must have a size almost equal to the design size even if the focusing or the amount of light applied (hereinafter called "exposure dose") differs from the design value. Second, they must have slits each having a rectangular cross section.
FIG. 1 shows the relation between the exposure dose and the developing speed of a resist film. The exposure dose is plotted on the abscissa, and the developing speed on the ordinate. As seen from FIG. 1, the developing speed is proportional to the exposure dose, once after it reaches a certain value. That is, the speed slopes at an angle .theta. with respect to the exposure dose. The dissolving characteristic .gamma. of the resist film is given as: .gamma.=tan .theta..
FIG. 2 represents the relation the exposure dose margin and the focusing margin have with respect to the dissolving characteristic .gamma.. The exposure dose margin is plotted on the abscissa, whereas the focusing margin is plotted on the ordinate. When .gamma.=2, the exposure dose margin and the focusing margin have the relation indicated by the solid line. When .gamma.=6, they have the relation indicated by the broken line. When .gamma.=12, they have the relation indicated by the dot-dashed line. As can be understood from FIG. 2, both the exposure dose margin and the focusing margin will increase if the value .gamma. is increased. Hence, it suffices to increase the value .gamma. in order to form a resist pattern that has a size almost equal to the design size, when the exposure dose or the focusing differs from the design value.
A resist film must be processed into a resist pattern that has slits each having a rectangular cross section. To this end, the resist film must be more insoluble with the developing solution toward its upper surface. Various methods of providing such a resist film are known. These methods are classified into two types. The first type is to use a special material to form such a resist film. The second type is to apply a developing solution to a film of naphtoquinone-novolac resist (generally used as g-line resist or i-line resist), thereby forming an insoluble top layer. The method of the second type, in which an insoluble top layer is formed, is indispensable to form a resist pattern that has slits each having a rectangular cross section. Any resist film made by this method is, however, found disadvantageous in that the exposure dose margin and the focusing margin decrease toward the lower surface of the resist film. This is inevitably because the developing speed increases toward the lower surface, too.
FIG. 3 is a schematic representation of the dissolving rate r.sub.1 in the upper surface of a resist film and the dissolving rate r.sub.2 in the entire resist film. Generally, r.sub.1 &lt;r.sub.2 as is shown in FIG. 3. The ratio n of r.sub.1 to r.sub.2, i.e., r.sub.1 /r.sub.2, influences the relation between the exposure dose margin and the focusing margin, as is illustrated in FIG. 4. In FIG. 4, the exposure dose margin is plotted on the abscissa, and the focusing margin on the ordinate. When n=0.2, the exposure dose margin and the focusing margin have the relation indicated by the solid line. When n=0.5, they have the relation indicated by the broken line. When n=1, they have the relation indicated by the dot-dashed line. As FIG. 4 shows, the greater the ratio n, the greater both margins. Hence, there are two requirements that should be satisfied to make slits having a rectangular cross section and to maintain both the exposure dose margin and the focusing margin at sufficient values. First, the resist should be improved in properties. Second, the rate of dissolving the resist film should vary with as the developing process proceeds.
Methods of varying the dissolving rate with time are disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 2-184373 and 4-314002. In these methods, a developing solution of high concentration is first applied to a resist film and a developing solution of low concentration is then applied to the resist film. However, it takes time to replace the unit for applying the high-concentration solution with the unit for applying the low-concentration solution. This makes it impossible to form slits having a rectangular cross section and to maintain both margins at sufficient values.
The dissolving rate may be varied with time by another method to develop a resist film having an insoluble top layer. In this method, a first developing solution of high dissolving power is first applied to a resist film and a second developing solution of low dissolving power is then applied to the resist film. That is, the method develops the resist film in two steps. In the first step, the resist film is developed with the first solution that can readily dissolve the insoluble top layer. In the second step, the resist film is developed with the second solution which differs in composition from the first solution and which has a smaller dissolving power than the first solution.
A method of this kind is disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 7-21640. In this method, the first unit for applying the high-concentration solution cannot be quickly replaced by the second unit for applying the low-concentration solution, either. In the case of a resist film having a high .gamma. value, such as a chemical amplification resist film, the developing will proceed to the lower surface of the film within one to two seconds. The first unit must therefore be replaced with the second unit within a time shorter than one second to maintain the exposure dose margin and the focusing margin at sufficient values. It is extremely difficult to replace the first unit with the second so fast, though it may seem easy to do so. As described above, a pattern should be formed so as to have a size almost the same as the design size even if the focusing or the exposure dose differs from the design value and to have slits each having a rectangular cross section. In a conventional method of forming a resist pattern, the resist film is first developed with a high-concentration solution and then with a low-concentration solution. In another conventional method, the resist film is first developed with a solution having high dissolving power and then with a solution having low dissolving power. With either conventional method it is impossible to replace one solution-applying unit with the other solution-applying unit within as short a time as desired.